Electrostatic printing process



May 17, 1966 T. FOHL ETAL ELECTROSTATIC PRINTING PROCESS 3 Sheets-Sheet 1 Filed July 10, 1961 SENSITIZED BENGAL PURE ZINCOXIDE y 1966 T. FOHL ETAL 3,251,687

ELECTROSTATIC PRINTING PROCESS Filed July 10, 1961 3 Sheets-Sheet 2 y 7, 1966 T. FOHL ETAL 3,251,687

ELECTROSTATIC PRINTING PROCESS Filed July 10, 1961 3 Sheets-Sheet 3 ZINC OXIDE SENSITIZED WITH ROSE BENGAL r cK '0 [h I PURE ZINC OXIDE T \i) I 4000A 5000A 6000A 7000A United States Patent 3,251,687 ELECTROSTATIC PRINTING PROCESS Timothy Fohl, Cambridge, Leonard E. Ravich, Brookline,

and Robert Lyon Spurr, Watertown, Mass, assignors to .Itek Corporation, Lexington, Mass, a corporation of Delaware Filed July 10, 1961, Ser. No. 123,038 -4 Claims. (Cl. 96-1) This invention relates to an improved electrostatic printing process.

The improved electrostatic printing process of the invention employs printing media, including media showing panchromatic response, which comprise substantially colorless, white-appearing, dye-sensitized zinc oxide pigments dispersed in a binder and applied to a suitable base. The media can be suitably compounded to have a variety of spectral response characteristics, and, in particular, can be compounded to show a peak sensitivity matched with the peak emissivity of a light source in the visible or ultra-violet region. For example, the media can be prepared to be maximally sensitive to the peak emissivity of a tungsten source at about 6000-6500 Angstroms.

These media are, further, particularly adaptable to electrostatic charging by a technique employing alternating current fields.

After charging, the media are exposed in a known manner to a light image, whereby those portion-s thereof exposed to light substantially lose-their electrostatic charge. The unilluminated portions of the exposed printing media are then particularly adaptable to subsequent development involving treatment with a liquid toner or developer comprising a suspending liquid having a softening action on the binder matrix of the media, a coloring pigment, and a substance imparting electrical charge to the pigment.

. The general principles of electrostatic printing are well known in the prior art. For example, prior art electrostatic printing processes commonly employ a printing medium comprising a photosensitive pigment, usually finely divided zinc oxide, incorporated in anon-conducting binder vehicle on a suitable base. In the prior art, an electrostatic charge is imparted to such a printing medium by means of a high voltage D.C. corona discharge. The charged medium is then exposed to a light image having light and dark portion-s to be reproduced. Those portions of the charged medium exposed to light become photoconductive and substantially lose their charge. Those portions of the medium not exposed to light retain an electrostatic charge in areas corresponding to the dark portions of the image. This latent electrostatic image on the medium is then developed, commonly by applying a charged dry, fusible particulate pigment over the exposed medium, whereby particles of the pigment are attracted v to the electrostatic charge remaining onthe unilluminated portion of the exposed medium and visibly reproduce the invisible charge image. Commonly, the image is then fixed in the medium by heating, whereby the finely divided pigment is fused onto the surface of the medium.

The prior art process as described has numerous disadvantages. For example, if white printing media are desired, they must be prepared with zinc oxide pigments which become photoconductive only when exposed in a certain limited, high-frequency, spectral range. Satisfactory exposure of such media necessitates the use of light sources having a strong component in the ultra-violet range, such as mercury-vapor lamps. Such light sources are expensive because of their special construction, and

are often bulky. Further, devices making use of such sources must be equipped with optical systems transmitting ultra-violet light, adding considerably to the cost of such devices.

The prior art, in attempting to increase the light sensitivity of zinc oxide-containing electrostatic printing media, has developed dye-sensitized media. However, the prior art dye-sensitized media are prepared in a manner said to render the light sensitivity of the media dependent upon the amount of dye employed in the media. Thus, the prior art incorporates a sensitizing dye, together with a zinc oxide pigment, in the binder matrix applied to a-suitable base. The resulting product contains a large quantity of dye in the binder, imparting a heavy tint thereto.

The tint of the printing papers produced in the prior art is said to be a forced compromise between the objectionable color imparted by the dye to the paper and the increased sensitivity said to be aiforded by increasing amounts of dye. However, it has been found that the presence of large amounts of dye in these media actually decreases their sensitivity, sinoe the excess dye acts as a filter absorbing light which would otherwise be effective in photosensitizing the zinc oxide particles.

The improved printing process of this invention makes use of substantially colorless dyessensitized zinc oxide pigments containing no dye in excess of that useful for photosensitization of the zinc oxide pigment. Thus, the applicants have dis-covered that zinc oxide can be sensitized with such minute amounts of dye as will impart little or no residual tint to printing media prepared from such sensitized pigments. The elimination of excess dye from the media increases their sensitivity by obviating filter efiects, and has permitted the development of media having a predetermined desirable photosensitive response, including substantially panchromatic media which are photosensitive over a broad portion of the visible spectrum (i.e.,

from about 4000 to about 7000 A.).

Although several theories are current concerning the mechanism of dye sensitization of photosensitive particles, it is believed that such sensitization involves excitation, by light, of electrons into the conduction band of the dye molecule. The electron then drifts into the conduction band of the photoconductor or other photosensitive particle on which the dye molecule is adsorbed. In such a mechanism, excess, unadsorbed dye does not afiect the photosensitivi-ty of the adsorbing particle, and may even be detrimental in reducing the sensitivity of the system by absorbing light which would otherwise reach adsorbed dye molecules.

In charging the printing media of the prior art, the art has usually employed a high voltage D.C. corona discharge. According to the present invention, it has been discovered that printing media, particularly the media of the present invention, can be suitably electrostatically charged by a corona discharge from a conduct-or connected to a source of alternating current. Since it is commonly accepted in the art that a uniform electrostatic charge must be imparted to the surface of an electrostatic printing medium before exposure, it is surprising that such a uniform electrostatic charge can be imparted to a printing medium using an alternating current source. The use of an alternating current source affords distinct advantages over the prior art process employing direct current, since bulky and expensive rectifying apparatus can be dispensed with.

Finally, the prior art method ofdeveloping the latent electrostatic image in a printing medium has numerous 3 disadvantages. The use of dry, charged pigment particles as the optically visible substance used to reproduce the latent electrostatic image is disadvantageous in requiring complex techniques such as magnetic brushing or the use of aerosols for application of the powders to the charged medium. The powders are difficult to contain and control, are messy, and contaminate devices in which they are employed. Because the particles become snagged on irregularities in the surface of the printing medium (e.g., paper fibers where paper is used as a backing), a background or unwanted darkening of exposed, uncharged areas of the image results. Also, development using dry particles requires that the particles be fixed onto the printing medium in separate steps after development to obtain any durability of the image. As noted, this fixing of dry particles to a medium is generally accomplished by fusing the particles at an elevated temperature. The need to employ fusible particles obviously limits the choice of particles used in the development process.

The use of liquid developers obviates these difliculties and also permits the sequential development or two or more images in the same medium. Multicolor prints can be made, for example, by a sequence of exposures of the same image, in register, through colored negatives, with intervening development after each exposure using appropriately colored developing pigments.

The prior art has heretofore employed liquid developing agents comprising charged particles in a suspending liquid. However, since random effects such as the triboelectric effect (i.e., development of electric charge of opposite sign on the particles, on the one hand, and the suspending liquid on the other due to frictional forces) have been relied upon as the source of particle charge, the art is limited to such systems of particles and suspending fluids as exhibit such random effects with impartation of charge of the proper sign to the particles. Also, for maximizing the triboelectric effect, the art is restricted to particular optimum ranges in particle size. Finally, in employing liquid developers of this type, the art still does not provide suitable means by which a developed image can be rendered permanent in the developed medium.

The improved electrostatic printing process of the present invention employs a liquid developer comprising:

(1) A liquid having a softening action on the binder matrix of the printing media employed;

(2) Finely divided opaque particles distributed throughout said liquid; and

(3) A substance imparting electrical charge to said particles.

The liquid, which functions as a carrier for the pigment particles the-rein, not only evenly distributes the particles to residually charged surface portions of a charged and exposed printing medium to develop the latent electrostatic image therein, but physically softens the binder of the medium so that the particles become fixed in the medium by development.

The particles employed in the developer are not limited in nature or size to such materials as will acquire a triboelectric charge in the developing fluid,- but may be particles which are uncharged or which develop a triboelectric charge opposite in sign to that heretofore thought necessary.

Charge is imparted to the suspending particles by the addition of a charge-imparting substance to the developer composition. The particles, acquiring a charge in the presence of this component, not only are thus made adaptable to the development process but form a highly stable dispersion in the carrying liquid because of the forces of mutual repulsion which they exhibit.

Referring first to the dye-sensitized zinc oxide particles used in the process of the invention, and to their manufacture, the dyes employed to sensitize zinc oxide according to the invention are organic materials able to transfer electrons into the conduction band of the oxide when illuminated with light of predetermined spectral characteristics, particularly visible light (e.g., from a tungsten source), thus rendering the oxide photoconductive outside its normal spectral range of response, which normal range is far toward the ultra-violet. This dye-sensitization, which results in the formation of particles of white zinc oxide having organic dye molecules adsorbed thereon, must be distinguished from the sensitization of zinc oxide by conversion of the white oxide to another form, commonly known as pink zinc oxide. The latter process involves treatment of pure white zinc oxide with reagents effecting chemical change of the starting material, and is exemplified in the teachings of US. Patents 2,727,807 and 2,727,808, among others.

The effect of dye sensitization, as referred to in the present specification, on the photoconductivity of zinc oxide is shown in FIGURE 1 of the accompanying drawings. FIGURE 1 shows the photoconductive response, on the ordinate, of unsensitized zinc oxide, and zinc oxide sensitized with Rose Bengal according to the invention, as a semilogarithmic function of wavelength plotted on the abscissa. The measurements were made by detecting, with an electrometer, the current conducted by the sample in the dark (dark current) and under illumination of monochromatic light (light current) from a monochromator. The ratio of the difference between the light and dark currents to the dark current, normalized by an efficiency factor for the monochromator is a pure number plotted on the ordinate. As is evident from FIGURE 1, the spectral response of the dye-sensitized sample extends far into the visible region (4000 A.7000 A.), whereas maximum sensitivity for the unsensitized sample is in the ultra-violet (below 4000 A.).

The dyes employed to sensitize zinc oxide according to the invention are colored organic materials, adsorbable on the oxide, which have an absorption peak or peaks outside the ultra-violet spectrum and in the visible spectrum. The dyes are, in general, those heretofore used in the photographic arts for sensitizing silver halides (cf. The Theory of the Photographic Process by C. E. Kenneth Mees, The Macmillan Publishing Company, New York, Revised Edition (1954) and include acridine dyes such as Acridine Orange (absorption peak at 4930 A.), thiazine dyes such as Methylene Blue (absorption peaks at 6678-6093 A.), phthaline dyes such as Rose Bengal (5600 A.) and Eosin Y (47004600 A.), cyanine dyes such as Kryptocyanine, and triphenylmethane dyes such as Malachite Green (6320 A.), Crystal Violet (5910- 5405 A.), and Alphazurine (6000-6500 A.).

It is believed that maximum sensitization of the dyetreated zinc oxide particles is obtained when a substantially monomolecular layer of the dye has been adsorbed on the surface of the zinc oxide particles. Only the dye thus adsorbed is capable of usefully transferring electrons to the zinc oxide particles: dye in excess of this quantity only interferes by absorbing light otherwise destined for the sensitized particles. The method of sensitizing the particles is, thus, to form such a dye monolayer, and then to remove excess dye from the particles. This method is to be contrasted with the prior art in which large amounts of dye are combined with the particles to be sensitized in the binder matrix of a printing medium, and are retained in the medium after its preparation.

In forming a monomolecular layer of dye on zinc oxide particles by contacting them with a dye and then removing any dye in excess of that required for the monomolecular dye layer, the sensitizing dyes are suitably applied to the zinc oxide particles to be sensitized by contacting the oxide particles with a solution of the sensitizing dye dissolved in a solvent for the dye. Suitable solvents are known to the art, and include polar solvents such as ethanol, methanol, acetone, and similar liquids having a dielectric constant greater than about 5 at 20 C.

In sensitizing the zinc oxide pigment, it is convenient to prepare a paste of the pigment with a solution of the dye. For example, 70 grams of zinc oxide particles hav- .ing an average diameter of 3 microns are suitably mixed with 100 cc. of a dye solution having a concentration from between f M to 10- M. Zinc oxide particles in this state of subdivision have a surface area of about t 3450 cm. /gm., so that in these solutions the ratio of the mols of dye employed to the surface area of the zinc oxide particles being treated varies between about 4 10 rnols/cm. to about 4X 10- mols/cm. Particularly good results are obtained when the dye/ surface area ratio is kept within this range.

The size of the zinc oxide particles being treated is not limited to that mentioned above. A number of zinc oxide powders are commercially available and are suitable for use in preparation of electrostatic printing media. The particles may range in size from an average particle diameter of about 0.1,u, having a surface area of about 10 10 cm. gm. to much larger particles having a sur face area of only about 2400 cm. gm. Numerous zinc oxide particles of this type are discussed, for example, in the article by Emmett et al., in Industrial and Engineering Chemistry, Analytical Edition, 13, 28-33 (1941). In sensitizing zinc oxide materials of the type described, it has been observed that optimum results in the dye sensitization process are suitably achieved when the ratio of the amount of dye contacted with the particles to the surface area of the particles being treated is about 10" mols/cm. Although suitable ratios of dye to surface area can be achieved by applying the dye in either diluted or concentrated solution, it has been observed that the dye is better adsorbed on the zinc oxide particles when the dye solutions are relatively concentrated, optimally around 10 M to 10 M.

It is desirable to allow the zinc oxide particles to remain in contact with the dye solutions for as long a period of time as is feasible, since adsorption is a time dependent process. At room temperature (25 C.), Rose Bengal and Eosin Y, for example, have been observed to be suitably adsorbed on zinc oxide particles from methanol solution in about 624 hours. The application of Methylene Blue or Acridine Orange in acetone is suitably allowed to proceed at room temperature for a period of time ranging from about 24 hours to one week. Sensitization can be achieved with the dyes mentioned if the dye solution and particles are allowedto remain in contact with each other for at least about 24 hours at room temperature.

A preferred method of sensitization, which shortens the time period of contact, proceeds by contacting the dye solution and zinc oxide particles at a temperature above room temperature, for example, at temperatures up to or above 100 C. Temperatures above 100 C. may also be employed providing they are not so high as to cause substantial deterioration of the dye employed. Practically, heating is generally accomplished at a temperature below about 200 C., such as at 150 C., in

'from 3 to 24 hours in appropriate containers.

The increase in adsorption brought about by use of elevated temperatures can be illustrated by reference to a treatment of 70 grams of zinc oxide in 100 cc. of a 10 M solution, in methanol, of Rose Bengal or Eosin Y. At 150 0., adsorption of either of these dyes is substantially complete in 3 hours; that of Methylene Blue is substantially complete in about 30 hours.

To remove any dye in excess of that forming a monomolecular dye layer on the particles, the particles are next washed to remove all but an adsorbed dye layer to give a substantially colorless zinc oxide product having adsorbed thereon a monomolecular layer of the sensitizing dye. For washing, a non-polar washing fluid suitably havinga dielectric constant of less than about 3 at C. may be used. Suitable materials of this type are solvents such as aliphatic, aromatic, and cycloaliphatic hydrocarbons, for example, hexane, heptane, octane, benzene, toluene, xylenes, cyclopentane, cyclohexane, etc. However, it is preferable to use as a wash fluid a polar material like that used in applying the dye to the oxide particles, providing care is taken that no more dye is removed than that in excess of the amount required to form a monomolecular layer on the particles.

A suitable was-hing procedure comprises removing the solution in which the dye is applied by filtration, and then Washing the zinc oxide particles in the. filter With the Washing fluid. In such a process, it is desirable to continue washing the dyed particles with washing fluid until the wash fluid is clear. To prevent fixation of excess dye on the sensitized particles and to obviate any cracking of the filter cake which would interfere with good Washing, it is desirable always to keep a layer of liquid over the surface of the dyed particles being washed. Generally, amounts of Washing fluid of about 80100 cc. are suflicient to remove excess dye from a 70 gram sample of sensitized zinc oxide, but more or less may be used to get a clear filtrate.

It is believed that the removal of excess dye in this manner is efiected by a physical displacement of the treati-ng solution containing excess dye. Dye in excess of that adsorbed on the zinc oxide particles to produce a substantially monomolecular layer is removed in the process.

After excess dye solution has been removed from the particles by Washing until the wash fluid is clear, the particles are thoroughly dried, such as by air drying at room temperature, preferably in a dust free atmosphere, or by moderate heating.

An electrostatic printing medium can be prepared from the novel dye sensitized zinc oxide particles above described by applying to a suitable backing material a coating comp-rising the sensitized photoconductive particles in combination with an electrically-insulating, lighttransmissive, transparent or translucent film-forming vehicle or binder, usually resinous. For example, such coatings may contain between about 1 part byweight of dye sensitized zinc oxide particles to one part by weight of resin solids and up to l6 parts by Weight of these zinc oxide particles per part of resin solids. Particularly good coatings are produced when the ratio by weight of particles to resin solids is about 5:3.

Suitable resin materials, for example, are the silicone resins now used in the art as binders for the photo-conductive particles of an electrostatic printing medium, modified according to the present invention as hereinafter described, but other materials now'used in the art can also be employed in the invention after suitable modification according to the present disclosure. In general, the conductivity of the binder should be about 10- rnhos/cm. or less.

Electrostatic printing media are prepared by intimately mixing the resin and a solvent therefor with the sensitized zinc oxide particles and then coating a suitable base with the resulting mixture. The thickness of the coating on the base may range from between about 0.5 milup to 4 mils in thickness, as known in the art. The coatings may be applied by techniques standard in the art, such as blade coating or flow coating. Suitable base materials having a smooth surface and moisture and temperature stable electrical properties are known in the art, e.g., aluminum foil or various conductive papers.

To obtain a binder matrix which will soften, but not dissolve, in the presence of the developer fluids described elsewhere herein, it is critical that a hardener be added to the resin binder in the preparation of electrostatic printing media according to the invention. The hardeners employed are, suitably, heavy metal salts of organic acids, i.e., soaps, such as linoleates, resinates, naphthenates, octoates, tallates, etc., of lead, cobalt, manganese, zinc, calcium, iron, and the like, and other hardeners of the type conventionally employed in paint technology (of. Organic Coating Technology, volume 1, by H. F. Payne, John Wiley and Sons, New York (1954), pages 227-240). Hardeners of this type are employed in such amounts that the weight of metal in the hardening agent used amounts to 0.3-l percent, optimally 0.5 percent by weight, based on the weight of the film-forming resin solid employed.

The incorporation of a hardener into the resin binder renders the binder softenable, but not readily soluble, in the liquid developers hereinafter described. During development, solid particles, present in the liquid developer and used to form a visible image on the exposed printing medium, are adhered to, or incorporated or embedded into the binder, softened by the liquid developer, at room temperature. The presence of a hardener in the binder also appears to promote quick rehardening of the binder after development so that the particles used to produce the image in the printing medium are tightly bonded to the rehardened resin coating.

Using zinc oxide particles sensitized with one of the dyes disclosed as suitable for sensitization, orthochromatic printing media photoconductive in the presence of light from any predetermined light source can be prepared. Such papers will all be suitable for the reproduction of black and white images, and of monochromatic images corresponding to the orthochromatic sensitivity of the medium. 7

To achieve optimum sensitivity of a printing medium to to visible light of several different wavelengths, or sensitive to light throughoutsubstantially the entire visible spectrum, a plurality of portions of zinc oxide particles, each portion sensitized with a different dye, can be combined. Alternatively, a single batch of zinc oxide particles can be contacted with a plurality of different dyes, either simultaneously or successively. Panchromatic media prepared with pigments sensitized as described above can be used for the monochromatic reproduction of polychromatic images such as color transparencies, for example.

In preparing media sensitive to a broad spectrum, it is preferred to combine a plurality of zinc oxide batches each contacted with a different dye and optimally sensitivein different, preferably overlapping, portions of the visible spectrum. The different batches may be combined in any relative proportions, but it is convenient to combine them in about equal parts by weight. By working with zinc oxide batches sensitized each with a different dye, maximum control of the sensitizing process, minimum interference of the dyes with each other, and maxi mum photoresponse are achieved.

However, an alternative method yielding sensitized material somewhat less sensitive than that described above can be made by contacting a single batch of zinc oxide with a single solution of a plurality of dyes. In preparing the solutions, adsorption curves of the dyes individually on zinc oxide are used to determine the concentration of each dye required in the mixed solution to give the desired proportion of dyes after adsorption.

Finally, a single batch of zinc oxide can be successively contacted with different dye solutions, with a removal of dye in excess of that required to form a monomolecular layer before contact with .the next dye solution. Since it involves many steps, this method is least easy to control.

Before exposure and development, the electrostatic printing media of the invention, as described above, are provided with an electrostatic surface charge by bringing the media into the vicinity of a negative corona discharge. The media are adaptable to charging, using either the charging techniques of the prior art which employ a source of direct current, or according to the charging process of the invention in which an alternating current source is used. In general, in the'A.C. charging process of the present invention, voltages similar to those used in the prior art D.C. charging processe can be employed (i.e., -7 kilovolt sources), although higher voltage sources are preferred. Ordinary 60 cycle current can be used, but is not critical, and other frequencies, par ticularly lower frequencies, are suitable.

FIGURE 2 shows schematically a device which has been employed with particular success for charging the printing media according to the invention. FIGURE 2 shows conductive plate 111 connected to one terminal of a high voltage transformer 12 into which is fed alternating currentfrom source 13. Variable resistance element 14 may be included in the circuit if convenient. The second terminal of transformer 12 leads to wire grid 15 mounted on insulating frame 16. For charging, the medium to be charged is simply placed on plate 11, between the plate and grid 15, for a time sufficient for the corona discharge emanating from grid 15 to charge the medium.

In a grid of the type 15 involving a multi-wire system, a spacing of at lea-st about /8 inch between the wires is preferred since otherwise the corona discharge may tend to emanate from only one or some of the wires, giving an irregular charge pattern. In forming such grids, tungsten wires of small diameter, for example one to three mils, are preferred as giving a high field gradient. The use of a high-melting metal such as tungsten avoids problems of localized heating, but is not critical to the success of the method.

Other grid patterns can be used in the practice of the AC. charging process of the invention. For example, charging grids comprising a single wire or two or three wires, spaced for example /2 inch apart and conveniently, but not critically, equal in length to the paper to be charged, can be employed. When charging using a corona discharge device having an area smaller than that of the electrostatic medium to be charged, the wires are moved over the surface of the medium with a sweeping motion, preferably maintaining a distance of at least about inch to prevent arcing between the grid and the conductive plate 11 of FIGURE 2. Under such conditions, a suitable charge is transferred to the electrostatic printing medium in about 15 to 30 seconds, using a 3 wire grid. Other grid configurations increase or decrease the time required to produce an electrostatic charge on the printing medium, as known in the art employing direct current sources.

The success of the alternating current charging method is unexpected, in view of the necessity that a substantial negative charge be produced on the electrostatic printing medium. It is believed that perhaps rectification of the alternating current corona discharge is worked by the zinc oxide particles in the printing medium at the surface thereof. It has also been proposed that negative charge is transferred to the printing medium by means of negatively charged atmospheric particles such as oxygen ions. For the ion charging mechanism, it is though-t that during the positive portion of the alternating current cycle not as many negative oxygen ions are discharged as are produced during the negative portion of the alternating current cycle, perhaps because of an affinity of the negative ions produced for the zinc oxide surface and/ or a failure of production of positively charged atmospheric particles.

Exposure of the charged plate then follows. Because the sensitivity of the dye-sensitized zinc oxide particles of the invention can be made to peak corresponding to the peak emissivity of any predetermined light source, a wide choice of light source is open to use. For example, the media can be conveniently prepared with substantially colorless dye-sensitized particles which become highly photoconductive on exposure to light from an ordinary tungsten filament lamp. The exposure time necessary to produce a latent electrostatic image in a charged medium by discharge of certain portions thereof varies, as known in the art, with the character and intensity of the light source, the characteristics of the image to be reproduced, the distance of the light source from the paper, and the specific characteristics of the electrostatic medium being exposed. To illustrate the good sensitivity of the printing media of the present invention, it has been observed that an exposure time as short as 0.2 second to a 200 watt tungsten filament projection bulb spaced at a disunder the trade name Coblax.

tance of 2-3 feet from a charged printing medium gives excellent expo-sures.

After exposure, the exposed printing medium is conta'cted with a liquid developer or toner. The toner, which comprises a non-polar non-conducting fluid acting as a softening agent for the resin coating of the printing medium, contains finely divided particles of some colored material suspended therein, which particles become bound to the softened resin. Commonly, carbon black is used as the particulate substance used to develop the image in the printing medium, but other pigments having a color other than black can equally well be use-d. For example, excellent copies in red, yellow, blue, etc, have been made using the organic and inorganic pigments commonly found in oil paints.

As known in the art, these pigments, including carbon black, are suitably suificiently finely divided to be adaptable to forming a stable suspension in the fluid suspending medium and to afford good image resolution. For example, good results have been obtained in the developing process of the invention using as a pigment a commercial carbon black sold under the trade name Mogul A. This is a fluffy carbon black of high resistivity having an apparent density of 12 pounds per cubic foot, a particle diameter of about 28 millimicrons as determined by the electron microscope, and showing a reading of 84 on the Nigrometer scale.

Unlike in the prior art, the pigment particles used in the present invention need not be of a fusible substance, nor need they be materials acquiring a triboelectric charge in the carrier fluid. To charge the particles of the present developers, a charge-imparting component is combined with the fluid dispersant and dispersed particles. The materials particularly suitable for this use comprise mixtures of asphaltic polycyclic hydrocarbons, largely aromatic, but having long straight chain aliphatic residues attached thereto, and an average molecular weight of about 1500. A portion of the molecule apparently has an affinity for particles in the dispersing fluid and effects the coating of the particles. Also, since the char-geimparting component acquires a charge, it is believed that the molecule may contain a polar group. Such materials suitably comprise viscous mixtures of submicroscopically divided solids suspended in very viscous oily materials. On dispersion, the materials will migrate to a negative electrode, indicating that they are positively charged when dispersed. Materials of this type are commercially available as lubricants, for example,

A commercial material of the Cob-lax type has a viscosity of 22450 SSU at 130 F., a viscosity of 1024 SSU at 210 F., and a flash point of 425 F.

Because materials of this type per se contain finely divided solid matter and exhibit a positive charge on dispersion, they can be employed in combination with a developer fluid alone,in the absence of any additional pigment such as carbon black. However, to enhance the contrast obtained in a developed electrostatic image, ad ditional pigment of the type described is usually employed. The charge-imparting substances combined in the dispersion apparently coat any particles added to the suspension and impart to the particles the positive charge which the charge-imparting substance bears in the dispersion, thus renderng the suspended added particles suitable for migration to negatively charged portions of a latent electrostatic image.

Charged, finely divided colored particles coated with charge-imparting substance are suitably suspended in a volatile non-polar non-conducting fluid, which is also suitably non-toxic and of low flammability, and which has a partial solvent or softening effect on the resin binder of the electrostatic medium being developed. For the resins commonly used in the art as the binders in electrostatic coatings, non-polar non-conducting hydrocarbon materials such as hexane, ligroin, kerosene, and the like are used to particular advantage as the suspending fluids. These fluids can be combined with non flammable volatile chlorinated and fluorinated fluids having a conductivity of about 10* mhos./cm. or less, such as perchloroethylene, carbon tetrachloride, or high boiling Freons such as Freon BF, which is CCl FCClgF, having a normal boiling point of 92.8 C. These materials, which speed the softening of resin binders when employed in a developer, and which also speed the fixing time of the image after development, are usually used to decrease the flash point of the principal liquid component of the developer. They can be used in the developer in amounts suflicient to raise the flash point to a desired value. As their proportion in the mixtures increases, the image density obtained in the development decreases, and a practical upper limit on their presence in the mixture is set only by the density which may be required in a given image.

The non-polar character of the developer suspending fluids insures that the materials, which have high resistivity (e.g. above 10 ohmcms.), will not decay the residual charge on the exposed electrostatic printing medium. The use of such fluids which have normal boiling points less than about C. promotes rapid drying of the developed prints, generally in about one minute at room temperature.

To prepare the developing agents of the invention, any colored particles to be employed in the developer, such as finely divided carbon black, are suitably intimately mixed with the charge-donating materials described above. As mentioned, using the positive chargeimparting agents of the invention, additional pigment can be omitted altogether. combine one part by weight of the coloring pigment used with up to about 5 or 6 parts by weight of the chargeimparting substance. The content of pigment particles can be increased to give a ratio of pigment to charging substance as low as 1:3.

Particularly good results have been obtained when 5 parts by weight of charge-imparting substance are used with one part by Weight of carbon black or other pigment, and the materials are blended by agitation.

After intimate admixture of the pigment and chargeirnparting substance, the resulting mixture is combined with the fiuid component of the developer. In general, it is desirable to have the developer as dilute as possible, with respect to the concentration of pigment particles, as is consistent with the production of an image having sharp contrast. More concentrated solutions may tend to develop a background or darkening of uncharged portions of the latent image in the exposed electrostatic printing medium due to a purely physical deposition of particles. In general, suitable developing agents accord ing to the present invention have been prepared by mixing about 6 grams of the 5 to 1 pigment mixture above described with 1 liter of suspending fluid. To discourage background formation, this relatively concentrated dispersion can be reduced to as low as A of this strength by addition of more fluid, with a sharp strong black image being produced. As mentioned earlier, it is preferred to work within the above-disclosed range at the more dilute concentrations.

After mixing with the solvent, the suspension is preferably allowed to stand for at least 24 hours. This appears to promote the formation of a stable electric charge on the suspended pigment particles. In general, the longer the developer is permitted to stand, the more increasingly sharp become the images subsequently produced in electrostatic prints by its use.

Development of an exposed printing medium is simply accomplished by contacting the medium having a latent electrostatic image thereon with the developer. [For example, the fluid may be rolled, sprayed, or wiped over the surface of the printing medium, or the medium can :be dipped into the developer. As noted earlier, the fluid portion of the developer has a softening action on the However, it is suitable to binder coatings of the printing medium. Although the coating of the medium can be soaked in the developer for a period of minutes without significant loss of the binder, the use of excess pressure should be avoided if the developer is applied by wiping the coating to combat any tendency to disrupt the softened coating.

Prints developed in this manner can be air-dried in room temperature in about a minute, or drying may be promoted by gentle heating, for example, with warm air. It is to be emphasized that no high temperature heating is necessary to fix the pigment particles producing the image on the printing medium, as is required with the fusible developing agents common in the prior art, nor need the liquid-developed image be transferred to an- --other obsorbent material.

Example 111 A dye-sensitized zinc oxide was prepared by forming a paste comprising 70 grams of Florence Green Seal 8 zinc oxide having an average particle diameter of about 3 microns with 100 cc. of a 10. M solution of Rose Bengal in methanol. The mixture was allowed to stand for about 24 hours at room temperature, with occasional agitation to ensure thorough dispersion.

The mixture was then filtered under vacuum in a Buchner funnel until nearly all of the dye solution had been removed. About cc. of a mixture of equal parts by volume of the dye solution and of toluene were then added to the zinc oxide paste in the filter. Without permitting the paste to become dry, about 100 cc. of toluene were gradually added to the filter, keeping the zinc oxide moist. The amount of toluene washing fluid used was sufficient in amount that the filtrate obtained was clear in color.

After all the washing fluid had passed through the filter, the vacuum was maintained for about 5 to 10 min utes to dry the zinc oxide cake thoroughly.

By comparing spectrophotometric transmission measurements of the original dye-sensitizing solution with measurements made on the filtrate to determine the solutions of Eosin Y, acetone solutions of Methylene Blue and Acridine Orange, ethanol solutions of Kryptocyanine, and solutions of Malachite Green and Crystal Violet.

Example 1 b A dye-sensitized zinc oxide was prepared by forming a paste comprising 70 grams of Florence Green Seal 8 zinc oxide having an average particle diameter of about 3 microns with 100 cc. of 10 M solution of Rose Bengal in methanol. The mixture was allowed to stand for about 24 hours at room temperature, with occasional agitation to ensure thorough dispersion.

The mixture was then filtered under vacuum in a Buchner funnel until all of the dye solution had been removed. It was then removed from the funnel and mixed with methanol so as to make a slurry. This slurry was then filtered as before. The process of mixing a slurry from the cake and filtering this slurry was repeated until the filtrate obtained became colorless. At this point, the vacuum was maintained for about 5 or 10 minutes thoroughly to remove excess liquid from the cake. The cake was then removed from the filter and allowed to dry in air for at least 24 hours prior to further treatment. I

Example 2 An electrostatic printing medium was prepared by combining 50 grams of dried sensitized zinc oxide particles produced by the methods shown in Example 1 with 28'grams of a silicone resin and xylene mixture containing 60 percent resin solids by weight, balance xylene, commercially available under the tradename SR82. The materials, together with a zinc-cobalt naphthenate hardener in such amount that the weight of zinc and cobalt therein amounted to 0.5 percent of the weight of resin solids employed, were put into a Waring type blender and mixed for 3 to 5 minutes. The resulting mixture was blade coated onto a conductive paper backing and dried.

Using the same amounts of zinc oxide, silicone resin, and hardener given above, a thin mixture. suitable for flow coating of a substrate material was prepared by the addition to the ingredients to be blended of grams of toluene. The mixture was blended as above described, and then applied by flow coating to a suitable backing material. For the preparation of low viscosity mixtures of the latter type, thinning fluids compatible with the resin can be added to adjust the mixture to any desired viscosity. The amounts of such thinning fluid added are not critical, since the fluids are later removed by volatilization. In practice, up to about grams of a thinning fluid can be suitably employed, for example, with the amounts of resin described above.

Example 3a Proceeding as in Examples 111 or 1b, a number of batches of zinc oxide particles sensitized with different dyes were mixed in the following proportions:

The resulting mixture was light gray in color and had good sensitivity. When used in preparing a printing medium, as in Example 2 above, the mixture showed panchromatic response better than commercial papers sensitized with Rose Bengal. The spectral response of a printing medium prepared with the mixed oxides is shown in FIGURE 3 of the drawings. Photosensitivity is determined as for FIGURE 1, using a tungsten source in the monochromator. The photosensitivity of pure zinc oxide and of zinc oxide sensitized with a 10- M solution of Rose Bengal to be comparable with commercial papers is also shown for purposes of comparison.

Example 312 Proceeding as in Example lb (employing an alcohol Wash for the dyed particles), a number of batches of zinc oxide particles sensitized with difierent dyes were prepared using the following dye solution:

Rose Bengal (10* M); Eosin Y (l0- M); Methylene Blue (5 x 10 M).

Example 4 A zinc oxide sample was 'dye sensitized in the manner of Example 1 by contacting with a solution of the following dyes in the concentration indicated. The relative adsorbed concentration of the dyes on the particles, as estimated from adsorption curves is also noted:

Relative Dye Solution Adsorbed Concentration Concentration Crystal Violet." 4.5 (10 M 1 Methylene Blue l 57 Rose Bengal- 0. 96 Acri dine range 7.7 M 1. 82 Eosin Y 6.3 (10 M 2. 64 Malachite Green 2.0 (IO- M 0.87

The material so prepared was less photosensitive than the mixture described in Example 2.

Example 5 A printing medium prepared as in Example 2 Was electrostatically charged by passing a three-wire grid attached to a six kilovolt alternating current source back and forth slowly over the surface of the coating for about -30 seconds in the dark.

The charged medium was then exposed to a black and white line image using a 0.2 second exposure and a 200 watt tungsten filament lamp at a distance of about 2.5 feetfrom the medium.

Example 6 ligroin, and an aliquot portion of the resulting suspen,

sion was diluted to of the original concentration. The developer solution was allowed to stand for at least 24 hours. No flocculation (that is, aggregation) of the carbon particles into visible aggregates was observed after a long period of time. After two or three days, if the solution is undisturbed, some concentration of color can be observed at the bottom of a transparent container containing the developer. This concentration, apparently caused by gravity, does not involve the formation of any sludge. The solution can be rendered uniform in color by the slightest agitation, which redisperses the suspended particles throughout the body of the suspending fluid.

Example 7 with the developing agent was passed over the surface of the exposed plate, without substantial pressure being used. The passage of the wet cloth over the surfaceimmediately developed the image in the printing medium.

The developed prints dried rapidly, in about 1 minute,

after removal from the developing fluid. After about one hour at room temperature, the image formed by the particles was completely resistant to attempts to remove the image, providing that the resin coating on the printing medium was not itself physically attacked or damaged.

Although specific embodiments have been herein shown and described, it is to be understood that they are illustrative and are not to be construed as limiting on the scope and spirit of the invention.

What is claimed is:

1. An improved electrostatic printing process comprising electr'ostatically charging surface portions of a substantially colorless printing medium comprising substantially colorless dye-sensitized zinc oxide particles dispersed as a separate phase in a hardened and rehardenable binder matrix therefor, said binder matrix being substantially free of dye, exposing the charged surface of the medium to a light image, whereby those portions of the charged surface illuminated by the image become photoconducting and substantially lose their charge, and then developing and fixing a visible image in the exposed medium by contacting the medium with a liquid developer comprising finely-divided particles, said particles being coated with a charge imparting substance consisting essentially of a mixture of asphaltic polycyclic hydrocarbons thereby rendering said so coated particles, attractable to residually charged portions of the exposed medium, said coated particles being suspended in a softening agent for said binder matrix, whereby said particles from the developer are bound to the softened binder, and then drying said medium, whereby said hardenable binder hardens.

2. A process as in claim .1 wherein said printing medium is panchromatic.

3. A process as in claim 1 wherein said printing medium is exposed by illumination with visible light.

4. The process of claim 1 wherein said softening agent is ligroin.

References Qited by the Examiner UNITED STATES PATENTS 2,857,271 10/1958 Sugarman 961 2,875,054 2/1959 Griggs et al. 96-1 2,891,911 6/1959 Mayer 25262..1 2,892,708 6/ 1959 Walkup 96-1 2,892,709 6/1959 Mayer 961 2,899,335 8/1959 Sta-ughan 961 2,907,674 10/1959 Metcalfe et a1 961 2,982,647 5/1961 'Ca-rlson et al. 96-1 2,993,787 7/1961 Sugarman 96-1 3,001,888 9/1961 Metcalfe et al. 25262.1 3,047,384 7/1962 Jones et al. 961 3,051,569 8/1962 Sugarman et al. 96--1 3,052,540 9/1962 Greig 96-1 3,069,365 12/ 1962 Shely 961 3,078,231 2/1963 Metcalfe et a1. 25262.1 3,079,342 2/1963 Insalaco 252-62.1

FOREIGN PATENTS 201,416 4/ 1956 Australia.

NORMAN G. TORCHIN, Primary Examiner.

HAROLD N. BURSTEIN, ABRAHAM H. WINKEL- STEIN, ALEXANDER D. RICCI, Examiners.

J. E, ALIX, A. LIBERMAN, C. E. VAN HORN,

Assistant Examiners. 

1. AN IMPROVED ELECTROSTATIC PRINTING PROCESS COMPRISING ELECTROSTATICALLY CHARGING SURFACE PORTIONS OF A SUBSTANTIALLY COLORLESS PRINTING MEDIUM COMPRISING SUBSTANTIALLY COLORLESS DYE-SENSITIZED ZINC OXIDE PARTICLS DISPERSED AS A SEPARATE PHASE IN AHRDENED AND REHARDENABLE BINDER MATRIX THEREFOR, SAID BINDER MATRIX BEING SUBSTANTIALLY FREE OF DYE, EXPOSING THE CHARGED SURFACE OF THE MEDIUM TO A LIGHT IMAGE, WHEREBY THOSE PORTIONS OF THE CHARGED SURFACE ILLUMINATED BY THE IMAGE BECOME PHOTOCONDUCTING AND SUBSTANTIALLY LOSE THEIR CHARGE, AND THEN DEVELOPING AND FIXING A VISIBLE IMAGE IN THE EXPOSED MEDIUM BY CONTACTING THE MEDIUM WITH A LIQUID DEVELOPER COMPRISING FINELY-DIVIDED PARTICLES, SAID PARTICLES BEING COATED WITH A CHARGE IMPARTING SUBSTANCE CONSISTING ESSENTIALLY OF A MIXTURE OF ASPHALTIC POLYCYLIC HYDROCARBON THEREBY RENDERING SAID SO COATED PARTICLES, ATTRACTABLE TO RESIDUALLY CHARGED PORTIONS OF THE EXPOSED MEDIUM, SAID COATED PARTICLES BEING SUSPENDED IN A SOFTENING AGENT FOR SAID BINDER MATRIX, WHEREBY SAID PARTICLES FROM THE DEVELOPER ARE BOUND TO THE SOFTENED BINDER, AND THEN DRYING SAID MEDIUM, WHEREBY SAID HARDENABLE BINDER HARDENS. 